Method for optical time division multiplex and apparatus thereof

ABSTRACT

The present invention inputs a signal synthesized an optical pulse with a variable-wavelength laser beam different in wavelength from it to a delay unit (S 1 ). The delay unit branches the signal to two optical signals, produces an optical path difference between them to afford a delay, synthesizes them again to generate a multiplexed optical signal, and minutely varies the optical path length of one of them (S 2 ) The present invention measures output variance of the delay unit on a variable-wavelength laser beam resulting from the minute variance (S 3 ), and controls the optical path difference so as to minimize output variance at a position where the output is a maximum or minimum, or is a specific value other than them (S 4 ). This stabilizes a phase difference between adjacent pulses of the multiplexed optical signal outputted from the delay unit ( 5 ) with a simple construction in optical time division multiplexing technology.

BACKGROUND

1. Technical Field

The present invention relates to an optical time division multiplexingcircuit, and more particularly to an optical time division multiplexingcircuit using a delay interferometer.

2. Related Art

Recently, optical communication technologies are in the limelight astechnologies for transmitting a large amount of information at a highspeed. Of them, an optical time division multiplexing technology isexpected to achieve high-volume and very fast transmission along withwavelength division multiplexing technology.

In the optical time division multiplexing technology, an optical timedivision multiplexing circuit is used. In a conventional optical timedivision multiplexing circuit, an optical pulse train outputted from apulse light source is branched to two, and the branch signals arerespectively inputted to optical modulators for modulation. Then, aspecific delay is afforded to one of the modulated optical signals via adelay unit, and synthesized with the other to generate a multiplexedoptical signal.

In this case, to achieve very fast communication, a phase differencebetween adjacent pulses must be stabilized by stably affording a correctdelay to an optical signal. However, since the propagation time of lightdepends on the influence of environmental temperatures and otherfactors, it has been difficult to stably afford a correct delay to anoptical signal (for example, refer to JP Patent No. 3508901).

SUMMARY

Therefore, a problem of the present invention is to stabilize a phasedifference between adjacent optical pulses with a simple construction inthe optical time division multiplexing technology.

To solve the above-described problem, the present invention is a timedivision multiplexing method that inputs an optical pulse to a delayunit, branches an input signal to two optical signals, produces anoptical path difference between the two branched optical signals toafford a delay between the two optical signals, and synthesizes theoptical signals again to generate a multiplexed optical signal Themethod stabilizes a phase difference between adjacent pulses of amultiplexed optical signal by inputting a signal generated bysynthesizing the optical pulse with a variable-wavelength laser beamdifferent in wavelength from the optical pulse to the delay unit,minutely varying the optical path length of one of the two branchedoptical signals, and controlling the optical path difference of thedelay unit so as to minimize the variance of output of the delay unit onthe variable-wavelength laser beam at a position where the output is amaximum or minimum, or is a specific value other than the maximum orminimum.

When controlling an optical path difference of the delay unit at aposition where the output of the delay unit on the variable-wavelengthlaser beam is the maximum, the method controls the optical pathdifference according to a phase difference φ between adjacent pulses ofthe multiplexed optical signal obtained by Expression 1 (n is aninteger, λ_(p) is the wavelength of the optical pulse, λ_(cw) is thewavelength of the variable-wavelength laser beam, c is light velocity,and ΔT is the interval between adjacent pulses of the multiplexedoptical signal (ΔT=ΔL/c, where ΔL is an optical path difference)):

Expression 1

$\varphi = {{2\pi \; c \times \Delta \; T \times \left( {\frac{1}{\lambda_{p}} - \frac{1}{\lambda_{cw}}} \right)} + {2n\; \pi}}$

When controlling an optical path difference of the delay unit at aposition where the output of the delay unit on the variable-wavelengthlaser beam is the minimum, the method controls the optical pathdifference according to a phase difference φ between adjacent pulses ofthe multiplexed optical signal obtained by Expression 2 (n is aninteger, λ_(p) is the wavelength of the optical pulse, λ_(cw) is thewavelength of the variable-wavelength laser beam, c is light velocity,and ΔT is the interval between adjacent pulses of the multiplexedoptical signal (ΔT=ΔL/c, where ΔL is an optical path difference)):

Expression 2

$\varphi = {{2\pi \; c \times \Delta \; T \times \left( {\frac{1}{\lambda_{p}} - \frac{1}{\lambda_{cw}}} \right)} + {\left( {{2n} + 1} \right)\pi}}$

When controlling an optical path difference of the delay unit at aposition where the output of the delay unit on the variable-wavelengthlaser beam is the specific value, the method controls the optical pathdifference according to a phase difference φ between adjacent pulses ofthe multiplexed optical signal obtained by Expression 3 (n is aninteger, m is a value from −1 to 1, λ_(p) is the wavelength of theoptical pulse, λ_(cw) is the wavelength of the variable-wavelength laserbeam, c is light velocity, and ΔT is the interval between adjacentpulses of the multiplexed optical signal (ΔT=ΔL/c, where ΔL is anoptical path difference)):

Expression 3

$\varphi = {{2\pi \; c \times \Delta \; T \times \left( {\frac{1}{\lambda_{p}} - \frac{1}{\lambda_{cw}}} \right)} + {\left( {{2n} + m} \right)\pi}}$

To solve the above-described problem, the present invention is anoptical time division multiplexing device including: a synthesizing unitthat synthesizes an optical pulse and a variable-wavelength laser beam;a delay unit that branches an output signal of the synthesizing unit totwo optical signals, produces an optical path difference between the twooptical signals to afford a delay between the two optical signals, andsynthesizes the optical signals again to output a multiplexed opticalsignal; an optical path length varying unit that minutely varies theoptical path length of one of the two optical signals of the delay unit;a filtering unit that takes out a signal component of thevariable-wavelength laser beam from an output signal of the delay unit;and a phase difference control unit that, when the optical path lengthof the one optical signal is minutely varied by the optical path lengthvarying unit, stabilizes a phase difference between adjacent pulses of amultiplexed optical signal outputted from the delay unit by operatingthe optical path length varying unit so as to minimize the variance ofoutput of a signal component of the variable-wavelength laser beam takenout by the filtering unit at a position where the output is a maximum orminimum, or is a specific value other than the maximum or minimum.

In the above-described device, when controlling an optical pathdifference of the delay unit at a position where the output of a signalcomponent of the variable-wavelength laser beam is the maximum, thephase control unit controls the optical path difference according to aphase difference φ between adjacent pulses of the multiplexed opticalsignal obtained by Expression 4 (n is an integer, λ_(p) is thewavelength of the optical pulse, λ_(cw) is the wavelength of thevariable-wavelength laser beam, c is light velocity, and ΔT is theinterval between adjacent pulses of the multiplexed optical signal(ΔT=ΔL/c, where ΔL is an optical path difference)):

Expression 4

$\varphi = {{2\pi \; c \times \Delta \; T \times \left( {\frac{1}{\lambda_{p}} - \frac{1}{\lambda_{cw}}} \right)} + {2n\; \pi}}$

When controlling an optical path difference of the delay unit at aposition where the output of a signal component of thevariable-wavelength laser beam is the minimum, the phase control unitcontrols the optical path difference according to a phase difference φbetween adjacent pulses of the multiplexed optical signal obtained byExpression 5:

Expression 5

$\varphi = {{2\pi \; c \times \Delta \; T \times \left( {\frac{1}{\lambda_{p}} - \frac{1}{\lambda_{cw}}} \right)} + {\left( {{2n} + 1} \right)\pi}}$

When controlling an optical path difference of the delay unit at aposition where the output of a signal component of thevariable-wavelength laser beam is the specific value, the phase controlunit controls the optical path difference according to a phasedifference φ between adjacent pulses of the multiplexed optical signalobtained by Expression 6 (n is an integer, m is a value from −1 to 1,λ_(p) is the wavelength of the optical pulse, λ_(cw) is the wavelengthof the variable-wavelength laser beam, c is light velocity, and ΔT isthe interval between adjacent pulses of the multiplexed optical signal(ΔT=ΔL/c, where ΔL is an optical path difference)):

Expression 6

$\varphi = {{2\pi \; c \times \Delta \; T \times \left( {\frac{1}{\lambda_{p}} - \frac{1}{\lambda_{cw}}} \right)} + {\left( {{2n} + m} \right)\pi}}$

Preferably, the delay unit includes: a beam splitter that branches anoutput signal of the synthesizing unit to a first branched opticalsignal and a second branched optical signal, emits the first branchedsignal to a first optical path, and emits the second branched signal toa second optical path; a first mirror that is provided in the firstoptical path and reflects the first branched signal to the beamsplitter; and a second mirror that is provided in the second opticalpath and reflects the second branched signal to the beam splitter. Thebeam splitter includes a delay interferometer that synthesizes the firstbranched signal inputted from the first mirror and the second branchedsignal inputted from the second mirror, and outputs a multiplexedoptical signal.

Still preferably, the optical path length varying unit includes apiezoelectric element that is attached to a back surface of one of thefirst and the second mirrors and vibrates the relevant mirror at apredetermined frequency in parallel to the first or the second opticalpath to which the mirror relates. The phase difference control unitincludes a detector that extracts a component of the frequency from thesignal component of the variable-length laser light taken out by thefiltering unit, and a differential amplifier that extracts a differencebetween output from the detector and an AC voltage to the piezoelectricelement, or an addition amplifier that extracts the sum of output fromthe detector and an AC voltage to the piezoelectric element.

The present invention stabilizes the operation of the delay unit byinputting a signal generated by synthesizing a variable-wavelength laserbeam and an optical pulse to the delay unit to perform feedback controlby use of a signal component of the variable-length laser beam, therebystabilizing a phase difference between adjacent pulses of a multiplexedoptical signal. Therefore, without the need to use fast electroniccircuits, a phase difference between adjacent pulses of a multiplexedoptical signal can be controlled and stabilized with a simpleconstruction.

Furthermore, the present invention can arbitrarily change a phasedifference by changing the wavelength of variable-wavelength laser beamin a state in which a phase difference between adjacent pulses of amultiplexed optical signal is stabilized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart of an optical time division multiplexing method ofone embodiment of the present invention;

FIG. 2 is a block diagram showing the construction of an optical timedivision multiplexing device of one embodiment of the present invention;and

FIG. 3 is a drawing for explaining feedback control operation by alock-in amplifier.

DETAILED DESCRIPTION

Hereinafter, embodiments of the present invention will be described withreference to the accompanying drawings. FIG. 1 is a flowchart of anoptical time division multiplexing method of one embodiment of thepresent invention. As shown in FIG. 1, the present invention inputs asignal generated by synthesizing an optical pulse with avariable-wavelength laser beam different in wavelength from it to adelay unit (S1 of FIG. 1). The delay unit branches the inputted opticalsignal to two optical signals, produces an optical path differencebetween the two branched optical signals to afford a delay between thetwo signals, and synthesizes the optical signals again to generate amultiplexed optical signal. At this time, the delay unit minutely variesthe optical path length of one of the two branched optical signals (S2of FIG. 1).

The method measures variance of output of the delay unit on avariable-wavelength laser beam that results from the minute variance ofthe one optical path length (S3 of FIG. 1), and based on a measuredvalue of the output variance, controls the optical path difference ofthe delay unit so as to minimize output variance at a position where theoutput is a maximum or minimum, or is a specific value other than them(S4 of FIG. 1). This stabilizes a phase difference between adjacentpulses of the multiplexed optical signal outputted from the delay unit(S5 of FIG. 1).

The present invention stabilizes a phase difference between adjacentpulses of the multiplexed optical signal. Together, provided that thewavelength of optical pulse is λ_(p), the wavelength ofvariable-wavelength laser beam is λ_(cw), the interval between adjacentpulses of the multiplexed optical signal is ΔT (ΔT=ΔL/c, where ΔL is anoptical path difference and c is light velocity), when controlling anoptical path difference of the delay unit at a position where the outputof the delay unit on the variable-wavelength laser beam is a maximum,the present invention controls the optical path difference to a desiredvalue according to a phase difference φ between adjacent pulses of themultiplexed optical signal obtained by Expression 7 (n is an integer):

Expression 7

$\varphi = {{2\pi \; c \times \Delta \; T \times \left( {\frac{1}{\lambda_{p}} - \frac{1}{\lambda_{cw}}} \right)} + {2n\; \pi}}$

When controlling an optical path difference of the delay unit at aposition where the output of the delay unit on the variable-wavelengthlaser beam is a minimum, the present invention controls the optical pathdifference to a desired value according to a phase difference φ betweenadjacent pulses of the multiplexed optical signal obtained by Expression8 (n is an integer):

Expression 8

$\varphi = {{2\pi \; c \times \Delta \; T \times \left( {\frac{1}{\lambda_{p}} - \frac{1}{\lambda_{cw}}} \right)} + {\left( {{2n} + 1} \right)\pi}}$

When controlling an optical path difference of the delay unit at aposition where the output of the delay unit on the variable-wavelengthlaser beam is a specific value, the present invention controls theoptical path difference to a desired value according to a phasedifference φ between adjacent pulses of the multiplexed optical signalobtained by Expression 9 (n is an integer, and m is a value from −1 to1):

Expression 9

$\varphi = {{2\pi \; c \times \Delta \; T \times \left( {\frac{1}{\lambda_{p}} - \frac{1}{\lambda_{cw}}} \right)} + {\left( {{2n} + m} \right)\pi}}$

FIG. 2 is a block diagram showing the construction of an optical timedivision multiplexing device of one embodiment of the present invention.As shown in FIG. 2, the present invention includes a first opticalcoupler 1 that synthesizes a variable-wavelength laser beam (wavelengthλ_(cw)) outputted from a first light source OS1 and an optical pulse(wavelength λ_(p)) outputted from a second light source OS2, and a delayinterferometer 2 that branches an output signal of the first opticalcoupler 1 to two optical signals, produces an optical path differencebetween the two optical signals to afford a delay between the twosignals, and synthesizes the optical signals again to generate amultiplexed optical signal.

Although, in this embodiment, any of optical paths connecting opticalelements to each other is formed from an optical fiber, the constructionof an optical path is not limited to this. Moreover, instead of thedelay interferometer, any delay unit having functions equivalent to itmay be used.

The delay interferometer 2 includes: a beam splitter 3 that branches anoutput signal b1 of the first optical coupler 1 to a first branchedoptical signal b2 and a second branched optical signal b3, emits thefirst branched signal b2 to the first optical path OP1, and emits thesecond branched signal b3 to the second optical path OP2; a first mirror4 that is provided in the first optical path OP1, and reflects the firstbranched signal b2 to a beam splitter 3; and a second mirror 5 that isprovided in the second optical path OP2, and reflects the secondbranched signal b3 to the beam splitter 3.

The beam splitter 3 synthesizes the first branched optical signal b2inputted from the first mirror 4, and a second branched optical signalb3 inputted from a second mirror 5, and outputs a multiplexed opticalsignal b4.

A piezoelectric element 6, attached to the back surface of the secondmirror 5, minutely vibrates the second mirror 5 at a predeterminedfrequency in parallel to the second optical path OP2 to which itrelates. By the second mirror 5 being minutely vibrated by thepiezoelectric element 6, the optical path length of the second opticalpath OP2 varies minutely.

Furthermore, a second optical coupler 7 is provided at the output sideof the delay interferometer 2, and one optical signal branched by thesecond optical coupler 7 is inputted to an optical filter (OF) 8. The OFpasses only light having a wavelength of λ_(cw), whereby only signalcomponents of a variable-wavelength laser beam of output signals of thedelay interferometer 2 are taken out.

The output of OF 8 is inputted to a lock-in amplifier (LIA) 10 via aphotodiode 9. The lock-in amplifier 10 extracts components offrequencies resulting from the variance of optical path length of thesecond optical path OP2 from signal components of thevariable-wavelength laser beam.

A differential amplifier 11 is connected to the output side of thelock-in amplifier 10. The differential amplifier 11 extracts thedifference between output from the lock-in amplifier 10 and an ACvoltage to the piezoelectric element 6.

A piezoelectric element driving unit 12 is connected to the output sideof the differential amplifier 11. The piezoelectric element driving unit12, according to an output signal of the differential amplifier 11,vibrates the piezoelectric element 6 so that the variance of the outputis minimum, at a position where the output of signal components ofvariable-wavelength laser beam is a maximum or minimum, or is a specificvalue other than them. Thereby, a phase difference between adjacentpulses of a multiplexed optical signal outputted from the delayinterferometer 2 is stabilized.

Although, in this embodiment, a difference between output of the lock-inamplifier 10 and the AC voltage to the piezoelectric element 6 isextracted by the differential amplifier 11, an addition amplifier may beused instead of the differential amplifier to extract the sum of outputfrom the lock-in amplifier 10 and the AC signal to the piezoelectricelement 6 for control.

Thus, according to the present invention, by performing feedback controlby the lock-in amplifier 10 using signal components of avariable-wavelength laser beam, operating points of the delayinterferometer 2 are controlled and stabilized, whereby a phasedifference between adjacent pulses of a multiplexed optical signal isstabilized.

The following describes feedback control operation by the lock-inamplifier 10 with reference to FIG. 3. In FIG. 3, a reference point of aphase difference is set in a point where output of a signal component ofa variable-wavelength laser beam is minimum, that is, a point B in whichthe signal component of the variable-wavelength laser beam is perfectlyreversed in phase. When the piezoelectric element 6 vibrates at afrequency ωm, the signal component of the variable-wavelength laser beamis modulated. In this case, at a point C that has a little longeroptical path difference than the point B, a sinusoidal signal having thesame frequency as the frequency ωm of the piezoelectric element 6 isoutputted from the lock-in amplifier 10. On the other hand, at a point Athat has a little shorter optical path difference than the point B, thesign of vibration is reversed. At the point B, no more than doublefrequency component 2ωm is outputted. Therefore, an output signal of thelock-in amplifier 10 can be used as an error signal to the piezoelectricelement 6. Feedback control is performed so that, at the point C, ΔLmoves in a direction in which ωm becomes smaller, and at the point A, ΔLmoves in a direction in which ωm becomes greater, whereby control isperformed so that the ωm component is always zero.

The optical time division multiplexing device of the present inventionstabilizes a phase difference between adjacent pulses of a multiplexedoptical signal. Together, provided that the wavelength of optical pulseis λ_(p), the wavelength of variable-wavelength laser beam is λ_(cw),the interval between adjacent pulses of the multiplexed optical signalis ΔT (ΔT=ΔL/c, where ΔL is an optical path difference and c is lightvelocity), when controlling an optical path difference of the delayinterferometer 2 at a position where the output of the delayinterferometer 2 on the variable-wavelength laser beam is a maximum, theoptical time division multiplexing device controls the optical pathdifference according to a phase difference φ between adjacent pulses ofthe multiplexed optical signal obtained by Expression 10 (n is aninteger)

Expression 10

$\varphi = {{2\pi \; c \times \Delta \; T \times \left( {\frac{1}{\lambda_{p}} - \frac{1}{\lambda_{cw}}} \right)} + {2n\; \pi}}$

When controlling an optical path difference of the delay interferometer2 at a position where the output of the delay interferometer 2 on thevariable-wavelength laser beam is a minimum, the optical time divisionmultiplexing device controls the optical path difference to a desiredvalue according to a phase difference φ between adjacent pulses of themultiplexed optical signal obtained by Expression 11 (n is an integer):

Expression 11

$\varphi = {{2\pi \; c \times \Delta \; T \times \left( {\frac{1}{\lambda_{p}} - \frac{1}{\lambda_{cw}}} \right)} + {\left( {{2n} + 1} \right)\pi}}$

1.-6. (canceled)
 7. A method for stabilizing a phase difference betweenadjacent pulses of a multiplexed optical signal, the method comprising:dividing an input signal into a first optical signal and a secondoptical signal at a delay unit; producing an optical path difference tocreate a delay between the first and second optical signals, wherein theoptical path difference is a difference between a first optical path forthe first optical signal and a second optical path for the secondoptical signal; varying a first optical path length of the first opticalsignal; synthesizing the first and second optical signals to generate amultiplexed optical signal; outputting the multiplexed optical signalfrom the delay unit; measuring an output change of the delay unit thatresults from the variance of the first optical path length; andcontrolling the first optical path length so as to minimize the outputchange of the delay unit.
 8. The method of claim 7, wherein, when theoutput of the delay unit is a maximum, the optical path difference isdefined by:$\varphi = {{2\pi \; c \times \Delta \; T \times \left( {\frac{1}{\lambda_{p}} - \frac{1}{\lambda_{cw}}} \right)} + {2n\; \pi}}$wherein φ is a phase difference between adjacent pulses of themultiplexed optical signal, wherein n is an integer, λ_(p) is thewavelength of the optical pulse, λ_(cw) is the wavelength of thevariable-wavelength laser beam, c is light velocity, and ΔT is theinterval between adjacent pulses of the multiplexed optical signal, andwherein ΔT=ΔL/c, where ΔL is an optical path difference.
 9. The methodof claim 7, wherein, when the output of the delay unit is a minimum, theoptical path difference is defined by:$\varphi = {{2\pi \; c \times \Delta \; T \times \left( {\frac{1}{\lambda_{p}} - \frac{1}{\lambda_{cw}}} \right)} + {\left( {{2n} + 1} \right)\pi}}$wherein φ is a phase difference between adjacent pulses of themultiplexed optical signal, wherein n is an integer, λ_(p) is thewavelength of the optical pulse, λ_(cw) is the wavelength of thevariable-wavelength laser beam, c is light velocity, and ΔT is theinterval between adjacent pulses of the multiplexed optical signal, andwherein ΔT=ΔL/c, where ΔL is an optical path difference.
 10. The methodof claim 7, wherein, when the output of the delay unit is a specificvalue other than a maximum or a minimum, the optical path difference isdefined by:$\varphi = {{2\pi \; c \times \Delta \; T \times \left( {\frac{1}{\lambda_{p}} - \frac{1}{\lambda_{cw}}} \right)} + {\left( {{2n} + m} \right)\pi}}$wherein φ is a phase difference between adjacent pulses of themultiplexed optical signal, wherein n is an integer, m is a value from−1 to 1, λ_(p) is the wavelength of the optical pulse, λ_(cw) is thewavelength of the variable-wavelength laser beam, c is light velocity,and ΔT is the interval between adjacent pulses of the multiplexedoptical signal, and wherein ΔT=ΔL/c, where ΔL is an optical pathdifference.
 11. The method of claim 7, further comprising inputting theinput signal into the delay unit, wherein the input signal is generatedby synthesizing an optical pulse with a variable-wavelength laser beam,wherein a variable-wavelength laser beam wavelength is different than anoptical pulse wavelength.
 12. The method of claim 7, wherein the outputchange of the delay unit corresponds to a change in a frequencycomponent of the multiplexed optical signal.
 13. An optical timedivision multiplexing device comprising: a delay unit configured todivide an input signal into a first optical signal and a second opticalsignal, further configured to produce an optical path difference betweenthe first and second optical signals to create a delay between the firstand second optical signals, wherein the optical path difference is adifference between a first optical path for the first optical signal anda second optical path for the second optical signal, and furtherconfigured to synthesize the first and second optical signals to outputa multiplexed optical signal; an optical path length varying unit thatvaries a first optical path length of the first optical signal; and aphase difference control unit configured to measure an output change ofthe delay unit that results from the variance of the first optical pathlength, and further configured to control the optical path lengthvarying unit to minimize the output change of the delay unit.
 14. Theoptical time division multiplexing device of claim 13, wherein, when theoutput of the delay unit is a maximum, the optical path difference isdefined by:$\varphi = {{2\pi \; c \times \Delta \; T \times \left( {\frac{1}{\lambda_{p}} - \frac{1}{\lambda_{cw}}} \right)} + {2n\; \pi}}$wherein φ is a phase difference between adjacent pulses of themultiplexed optical signal, wherein n is an integer, λ_(p) is thewavelength of the optical pulse, λ_(cw) is the wavelength of thevariable-wavelength laser beam, c is light velocity, and ΔT is theinterval between adjacent pulses of the multiplexed optical signal, andwherein ΔT=ΔL/c, where ΔL is an optical path difference.
 15. The opticaltime division multiplexing device of claim 13, wherein, when the outputof the delay unit is a minimum, the optical path difference is definedby:$\varphi = {{2\pi \; c \times \Delta \; T \times \left( {\frac{1}{\lambda_{p}} - \frac{1}{\lambda_{cw}}} \right)} + {\left( {{2n} + 1} \right)\pi}}$wherein φ is a phase difference between adjacent pulses of themultiplexed optical signal, wherein n is an integer, λ_(p) is thewavelength of the optical pulse, λ_(cw) is the wavelength of thevariable-wavelength laser beam, c is light velocity, and ΔT is theinterval between adjacent pulses of the multiplexed optical signal, andwherein ΔT=ΔL/c, where ΔL is an optical path difference.
 16. The opticaltime division multiplexing device of claim 13, wherein, when the outputof the delay unit is a specific value other than a maximum or a minimum,the optical path difference is defined by:$\varphi = {{2\pi \; c \times \Delta \; T \times \left( {\frac{1}{\lambda_{p}} - \frac{1}{\lambda_{cw}}} \right)} + {\left( {{2n} + m} \right)\pi}}$wherein φ is a phase difference between adjacent pulses of themultiplexed optical signal, wherein n is an integer, m is a value from−1 to 1, λ_(p) is the wavelength of the optical pulse, λ_(cw) is thewavelength of the variable-wavelength laser beam, c is light velocity,and ΔT is the interval between adjacent pulses of the multiplexedoptical signal, and wherein ΔT=ΔL/c, where ΔL is an optical pathdifference.
 17. The optical time division multiplexing device of claim13, wherein the delay unit comprises a delay interferometer, and whereinthe delay interferometer comprises: a beam splitter configured to dividethe input signal into the first optical signal and the second opticalsignal, further configured to emit the first optical signal along thefirst optical path and emit the second optical signal along the secondoptical path; a first mirror in the first optical path configured toreflect the first optical signal back to the beam splitter; and a secondmirror in the second optical path configured to reflect the secondoptical signal back to the beam splitter, and wherein the beam splitteris further configured to synthesize the first optical signal inputtedfrom the first mirror and the second optical signal inputted from thesecond mirror, and further configured to output the multiplexed opticalsignal.
 18. The device of claim 13, further comprising a filtering unitconfigured to extract a signal component from the multiplexed opticaloutput signal of the delay unit.
 19. The optical time divisionmultiplexing device of claim 18, wherein the optical path length varyingunit comprises a piezoelectric element attached to a back surface of oneof the first or the second mirror, and wherein the piezoelectric elementis configured to vibrate the first or the second mirror at apredetermined frequency.
 20. The optical time division multiplexingdevice of claim 17, wherein the phase difference control unit comprises:a detector configured to extract a component of a frequency from thesignal component of the multiplexed output signal extracted by thefiltering unit; and a differential amplifier configured to determine adifference between an output of the detector and an AC voltage to thepiezoelectric element.
 21. The optical time division multiplexing deviceof claim 20, wherein the predetermined frequency is determined based onthe difference between the output of the detector and the AC voltage tothe piezoelectric element.
 22. The optical time division multiplexingdevice of claim 19, wherein the phase difference control unit comprises:a detector configured to extract a component of a frequency from thesignal component of the multiplexed output signal extracted by thefiltering unit; and an addition amplifier that determines a sum of anoutput from the detector and an AC voltage to the piezoelectric element.23. The optical time division multiplexing device of claim 13, furthercomprising a synthesizing unit configured to synthesize an optical pulseand a variable-wavelength laser beam to form the input signal.